RegisterSchema per golang package #490: include all schema nodes (#545)

Fixes #490 after
2a85fea
collected the node IDs incorrectly.

This uses the list of IDs after `resolveName()` has recursively
scanned the nodeMap and gathered all the IDs in the capnp file.
The previous commit used only the top-level file ID.

For example,
https://github.com/capnproto/go-capnp/blob/main/flowcontrol/internal/test-tool/writer.capnp.go
now gets generated with:

```
func RegisterSchema(reg *schemas.Registry) {
	reg.Register(&schemas.Schema{
		String: schema_aca73f831c7ebfdd,
		Nodes: []uint64{
			0xf82e58b4a78f136b,
		},
		Compressed: true,
	})
}
```

This commit fixes the above generated code, and includes the following:

`0x80b8cd5f44e3c477,`

`0xd939de8c6024e7f8,`

capnpc-go/nodes.go

@@ -275,10 +275,8 @@ func makeNodeTrees(req schema.CodeGeneratorRequest) (nodeTrees, error) {
 		f.pkg = ann.Package
 		f.imp = ann.Import
 		nnodes, _ := f.NestedNodes()
-		ids := make([]uint64, nnodes.Len())
 		for i := 0; i < nnodes.Len(); i++ {
 			nn := nnodes.At(i)
-			ids[i] = nn.Id()
 			if ni := ret.nodes[nn.Id()]; ni != nil {
 				nname, _ := nn.Name()
 				if err := resolveName(ret.nodes, ni, "", nname, f); err != nil {
@@ -294,7 +292,9 @@ func makeNodeTrees(req schema.CodeGeneratorRequest) (nodeTrees, error) {
 		}
 		// Note that this can collect ids from multiple files if they are in the
 		// same $Go.package.
-		pkg.nodeId = append(pkg.nodeId, ids...)
+		for _, n := range f.nodes {
+			pkg.nodeId = append(pkg.nodeId, n.Id())
+		}
 		ret.pkgs[f.pkg] = pkg
 	}
 	return ret, nil